1. Field of the Invention
This invention relates to riser tensioning systems on floating platforms and, more particularly, to an improved riser tensioning system which automatically compensates for tidal action or the like.
2. Description of the Prior Art
Existing riser tensioners being fixedly mounted on the floating vessel must compensate for both heave, tidal actions, vessel movement and the factors which change the distance between the well hole and the vessel. As a result tensioner compensating cylinders are usually provided with a forty foot stroke as compared to smaller twenty feet strokes for drill pipe heave compensators which are more easily adjusted for tidal compensation. As an alternative, some existing riser tensioners obtain the necessary travel by using four part cable reeving that multiplies a ten foot stroke of a ram to a forty foot cable take-up capability. These cables passing around four sets of sheaves are subjected to constant working and bending stresses and wear out in a short period of time requiring expensive replacement and expensive down time of the equipment while the cables are being replaced. In addition, the sheaves necessary for supporting these cables add undesirable weight to the vessel. Still further these systems with their four part sheaves at the end of the cylinder are positioned high in the vessel raising higher on the vessel the moment of the pull force from the riser reducing the stability of the vessel.